The extent and type of boundary directly affect numerical simulation results of CO2 geological sequestration in “infinite” deep saline aquifers thereby causes evaluation error of storage potential. In this paper, a typical 2D radial flow model is employed to analyze the effects of the extent and type of boundary on pressure buildup and CO2 saturation plume distribution, the maximum pressure buildup and CO2 migration distance after injecting CO2 into the deep saline aquifer using TOUGH2-MP software. To illustrate the relation between boundary effects and formation parameters, sensitivity analyses for permeability, compressibility, temperature, salinity, and boundary type are performed. The results indicate that the greater boundary extent leads to the more “average” pressure buildup and CO2 saturation, the smaller maximum pressure buildup, the more flat CO2 saturation distribution shape and the greater CO2 migration distance under no-flow boundary condition. But the boundary effects on storage forms are very small and the volume storage efficiency in the “infinite” deep saline aquifer is overestimated. The difference becomes smaller and smaller with the expansion of the boundary extent. In addition, according to analytical theory of single-phase and multiphase flow, one simple approximate analytical method for determining the boundary extent under a known injection rate is obtained. This method is applicable to boundary extent determination for single well or multi-well injection in the “infinite” deep saline aquifer and the maximum CO2 migration distance and effect boundary of single water phase is taken into account.